The present invention relates generally to wooden frame construction materials and techniques, and more specifically to an open-web wooden truss with enhanced fire resistance.
Since their introduction in about 1960, light timber, open-web wood trusses have become one of the most widely used engineered wood building products employed in commercial construction. According to the Wood Truss Council of America (WTCA), such trusses are lightweight, easy to install, and have nailable chords for easy attachment of roof decking and ceiling materials. Open-webbing provides great benefits to plumbers and electricians, without the need to spend time cutting holes in floor members. Less cutting reduces jobsite labor and reduces potentially critical errors that could result in compromising the structural integrity of the components. Open-web wood trusses are lighter, less expensive and can be stronger than large, single “closed web” support members.
When subject to fire damage, the weak link or cause of failure of such open-web trusses is the detachment of the metal gusset plates used to connect the framing members together. Under load, as the wood chars and the metal gusset plates heat up under fire, the teeth of the metal gusset plates lose strength and holding power. The loss of the gusset plate on the bottom chord of a truss can lead to tensile forces pulling the truss apart. The loss of a gusset plate on the top chord will cause any web members attached to the top chord to pull away. Both situations will significantly reduce the load-carrying capacity of the installed truss and may even lead to a truss collapse.
Thus, engineered building components provide adequate strength under normal loading, but under fire conditions, these truss systems can fail, leading to the collapse of roof, floors, and possibly the entire structure. Truss systems are usually hidden, and fires within truss systems can go undetected for long periods of time, resulting in loss of structural integrity prior to discovery of the fire. Structural design codes often do not factor in the decreased system integrity as the fire degrades the structural members. Accordingly, there is a need for open-web truss systems having enhanced fire resistance.